1. Field of Invention
The invention pertains to wavelength-agile laser transmitters for wavelength-division-multiplexed (WDM) optical communications networks. More particularly, the invention pertains to laser transmitters with internal wavelength referencing that can be remotely switched to arbitrarily selectable channels on a standard grid, and to re-configurable optical communications networks.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level time division multiplexed public switched telephone network (PSTN) signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. Therefore, WDM is the optical equivalent of frequency division multiplexing (FDM). Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 0.4 nm, i.e., 50 GigaHertz. At this separation, as many as 128 channels may be carried by a single fiber in a bandwidth range within the same capacity of an erbium doped fiber amplifier (EDFA). Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission.
The lasers presently deployed are distributed-feedback (DFB) diode lasers. The wavelength grid is defined by the transmission peaks of a Fabry-Perot reference etalon. The technology for fabricating these etalons from glass or fused silica in large quantities at reasonable cost is well developed. Present transmitters use a combination of a DFB laser and a reference etalon in a feedback control loop. DFB lasers suffer from the drawback that they have small tuning ranges of xcx9c3 nm (See, e.g., J. Carroll, J. Whiteaway, and D. Plumb, Distributed feedback semiconductor lasers, SPIE Press, 1998). A DFB laser""s characteristics can only be approximately determined by design. In practice the center wavelengths are distributed across the grid span due to statistical variations in the manufacturing process. Thus each DFB laser transmitter must be individually sorted and is usable for only one channel or a small number of adjacent channels. As a result, DFB laser transmitters cannot be assigned to arbitrary channels.
On the other hand, WDM networks are evolving towards re-configurable architectures in which each transmitter""s wavelength must be re-selectable by remote command. Re-configurable networks offer significant capacity, reliability, and management advantages over static systems (See, e.g., R. Ramaswami and K. Sivarajan, Optical Networks, A Practical Perspective, Morgan Kaufmann Publishers, 1998).
Thus there exists a need for laser transmitters that can be arbitrarily switched to desired channel wavelengths within the wavelength grid of a WDM network.
The present invention provides a tunable external cavity laser (ECL) with a compact form factor and precise tuning to any selected wavelength of a predetermined set of wavelengths comprising a wavelength grid. The ECL may thus be utilized in telecom applications to generate the center wavelengths for any channel on the ITU or other optical grid. The ECL does not require closed loop feedback to maintain wavelength accuracy. A novel tuning mechanism is disclosed which provides for electrical or mechanical tuning to a known position or electrical parameter, e.g., voltage, current, capacitance, with the required precision in the selected center wavelength arising as a result of a novel arrangement of a grid generator and a channel selector. The grid generator exhibits first pass bands which correspond to the individual channels of a selected wavelength grid and a finesse which suppresses cavity side modes of the laser. The channel selector exhibits second pass bands differing from the first transmission bands by an amount corresponding substantially inversely with the number of channels of the selected wavelength grid. This relationship allows xe2x80x9cvernierxe2x80x9d tuning of the output wavelength of the laser to a selected wavelength on the wavelength grid. The finesse of the grid generator and channel selector is chosen to suppress channels adjacent to the selected channel.
In an embodiment of the invention a tunable external cavity laser tunable to each selected center wavelength of a number of channels each centered on a corresponding gridline of a selected wavelength grid is disclosed. The tunable external cavity laser includes a gain medium, a grid generator and a channel selector. The gain medium to emit a beam. The grid generator is located in an optical path of the beam, and the grid generator of a first selected optical path length determinative of a first free spectral range substantially corresponding to a spacing between adjacent gridlines of the selected wavelength grid. The channel selector is located in the optical path of the beam and the channel selector with a tunable second selected optical path length determinative of a second free spectral range differing from the first free spectral range by an amount corresponding substantially inversely with the number of channels of the selected wavelength grid and said channel selector tunable within the selected wavelength grid to tune the beam to a selected one of the number of channels of the wavelength grid.
In an alternate embodiment of the invention the tunable external cavity laser includes the gain medium, a first filter and a tunable filter. The gain medium to emit a beam. The first filter is located in an optical path of the beam The first filter exhibits first pass bands substantially corresponding to the channels of the selected wavelength grid. The tunable filter is located in the optical path of the beam. The tunable filter exhibits second pass bands differing from the first pass bands by an amount corresponding substantially inversely with the number of channels of the selected wavelength grid. The tunable filter is tunable within the selected wavelength grid to tune the beam to a selected one of the number of channels of the wavelength grid.
In an alternate embodiment of the invention a gain medium operable as a grid generator is disclosed.
In an alternate embodiment of the invention a method for tuning a beam emitted by a gain medium to a corresponding center wavelength for each of a plurality of channels of a selected wavelength grid is disclosed. The method for tuning comprising the acts of:
filtering the beam to provide feedback to the gain medium of a first set of wavelengths substantially corresponding with the center wavelengths for each of the number of channels;
tuning the beam to provide feedback to the gain medium of a selected one of a second set of wavelengths with a periodicity differing from the periodicity of the first set of wavelengths by an amount corresponding substantially inversely with the number of channels of the selected wavelength grid.